What is Depth of Field
Set the right combination of aperture and shutter speed and you'll notice an immediate difference in your photography.
The depth of field is not an abrupt change, but a smooth gradual sharp to blurry transition, as can be seen in the photograph below. Everything in front or behind the focus plane begins to lose sharpness, even if it is not detectable by the camera or our eyes.
The depth of field should not be confused with Depth of Focus. The depth of Focus, or Focal Spread, describes the distance over which light is focused at the camera's sensor, as opposed to how much of the subject is in focus. It is important because it sets tolerances on how level the camera's sensor has to be in order to capture proper focus in all regions of the image.
Aperture and f Numbers
An aperture is simply a hole in the lens - a variable diaphragm which can be made larger or smaller to control how much light reaches the sensor. You can control the aperture size using the dial on your DSLR, or it can be set automatically by the camera.
Aperture is measured in f stops or f numbers and the numbers represent the ratio between the focal length of the lens and the diameter of the aperture. These f numbers are typically written as f/2.8 or f/22. A lens is said to be "wide open" when the aperture is at its largest, which is the lowest f number the lens is capable of. On average this is around f/5. A "closed" lens would be at the smallest possible aperture, typically around f/32.
The relationship between the numbers on the f-stop scale can be hard to grasp. Generally, f/4 is twice as large as f/8, which is twice as large as f/16. Each f-stop halves the amount of light reaching the sensor, meaning you have to double the shutter speed to compensate. The image below shows how the f-number affects the aperture.
Controlling Depth of Field
Aperture and focal distance are the two main factors affecting depth of field. As a general rule:
- Larger Apertures (smaller f-stop numbers e.g. f/2.8) will result in small DoF.
- Close focal distance will result in small DoF e.g. Macro.
- Smaller Apertures (larger f-stop numbers e.g. f/32) will result in a large DoF.
- Large focal distance will result in large DoF e.g. Wide-angle.
Depth of Field in Action
You can see in the animation below how the variation in aperture (f-number) changes how much of the scene is in focus. The larger the f-number the more of the scene is focused, while lower f numbers only the chess pieces focused on are in focus.
In this video, we can see how changes in aperture from wide open to closed affect the depth of field in the classic game of chess photograph. You can see how more of the chess pieces become in focus as the aperture decreases (higher f numbers).
Uses of Depth of Field
A wide aperture if f/2.8 will produce an image with a vry shallow depth of field, meaning that everything behind or in front of your focal point will be blurred, which is great for portraits. On the other hand, a narrow aperture, f/22 for example, will maximise the depth of field which is ideal for landscapes.
Depth of field also varies depending on the focal length of your lens and how close you are to your subject. The longer the lens and the closer you are to your subject, the shallower the depth of field at any given aperture.